Heretofore, polyester fibers have been used in various applications due to their excellent properties. But, polyester fibers for industrial applications are often used under severe conditions of high temperature and high humidity from the properties of their applications, and their properties are not satisfactory.
Polyester fibers, especially, for a dryer canvas which is used in a drying process in papermaking, a steel washing blush or the like are required to have sufficient durability for the long use under high temperature and high humidity. However, polyester fibers have had problems, namely in a high-temperature atmosphere in the presence of water, polyesters, especially polyethylene terephthalate, are apt to decompose, and accordingly to lower the degrees of polymerization to weaken a breaking strength, a knot strength and the like of the fibers, and they can not be used for a long time under high temperature and high humidity.
In order to solve these problems, various methods have been studied. For example, Japanese Examined Patent Publication No. 47-15104 proposes a process wherein the concentration of carboxyl terminal groups of polyethylene terephthalate is lowered by adding a combination of a copper salt of a carboxylic acid and a reductive anion. However, this process has such a disadvantageous point that when the above-mentioned stabilizer is added, the polyester is colored in an unfavorable color tone, and further, sufficient durability is not always obtained under severe conditions of high temperature and high humidity.
On the other hand, it is widely known that the use of polyethylene naphthalate, which has naphthalene rings in the molecular skeleton, in stead of polyethylene terephthalate can improve the durability of the fiber.
For example, there are Japanese Examined Patent Publication No. 47-49769, Japanese Examined Patent Publication No. 47-49770, Japanese Examined Patent Publication No. 56-42682, Japanese Unexamined Patent Publication No. 4-100914 and Japanese Unexamined Patent Publication No. 4-194021 regarding polyethylene 2,6-naphthalate fiber, and they describe that polyethylene naphthalate fibers having excellent mechanical properties and thermal stability are produced by specifying the conditions of melt spinning of polyethylene 2,6-naphthalate.
It is known that the polyethylene 2,6-naphthalate fiber can have a higher elastic modulus and tensile strength than the polyethylene terephthalate fiber, which is used widely conventionally, and the polyethylene 2,6-naphthalate fiber is good in resistance to wet heat because it has a high glass transition temperature. As a method for further improving the resistance to wet heat, for example, Japanese Unexamined Patent Publication No. 50-95517, Japanese Unexamined Patent Publication No. 56-85704 and the like disclose methods for reducing concentrations of carboxyl terminal groups of polyesters by adding a carbodiimide compound. However, when these methods are applied, hydrolytic decomposition resistance is improved, but whitening, cracking and fibrillation on bending, which are problems characteristic to naphthalate-based polyester fibers, still occur and proceed not only under a wet heat condition but also under a dry heat condition. These methods therefore are not effective for solving these problems.
That is, since polyethylene 2,6-naphthalate has a rigid molecular chain and a characteristic crystal structure, whitening is apt to occur on a bent part when the fiber is subjected to deformation accompanied by bending such as knotting or the like, and fibrillation and cracking proceed from the whitened part, and resultingly a knot strength and a loop strength are lost especially in the fiber which has a single fiber fineness of 10 denier or more, or which has been kept under high temperature and high humidity for a long time. This has remained as a problem.